We have demonstrated that follicular granulosa cells produce plasminogen activator in a manner closely correlated with ovulation. The substrate for this enzyme, plasminogen, is present in follicular fluid. The product of the reaction, plasmin, is a serine protease that can weaken the follicle wall. FSH, prostaglandins of the A, B and E series and cyclic adenosine monophosphate (cAMP) stimulate synthesis and secretion of the activator by granulosa cells in vitro. These studies suggest a role for plasminogen activator in ovulation and indicate that the cultured granulosa system is well suited for studies of the mechanism of hormone action. Furthermore, the sensitivity of the cells to FSH, and the ease with which the response can be measured makes it a convenient and useful in vitro bioassay for FSH. In the future it is planned to further define the relationship of plasminogen activator and ovulation. With the in vitro system, the nature of the relationship between FSH binding, cAMP production and plasminogen activator synthesis and secretion will be examined. Finally, the cultured granulosa cell system appears ideal for an examination of other aspects of the mechanism of hormone action and some of these questions will be addressed in the coming year.